Semiconductor optical amplifiers are known in which light is transmitted through a semiconductor waveguide by electrical stimulation similar to laser operation. Semiconductor material used in such a waveguide is herein referred to as active material. Typically it will include semiconductor materials from Group III and Group IV. Such active semiconductor materials may be electrically pumped in known manner to cause optical signal amplification. The semiconductor device may be formed as a silicon chip with an integrated waveguide of the type shown in our UK Patent 2307786.
Semiconductor optical amplifiers may be grown on the same crystal substrate as other elements of an optoelectronic integrated circuit. In such a case monolithic integration is achieved. However, other semiconductor optical amplifiers involve hybrid integration where a semiconductor optical amplifying chip is attached to an optoelectronic integrated circuit by mechanical means. This permits the optical amplification chip to be made from material dissimilar to that of the remaining optoelectronic integrated circuit. The integrated circuit device may include waveguides formed from silicon, silicon dioxide, polymer or other materials.
Known semiconductor optical amplifiers may consist of a semiconductor chip of active material arranged so that the optical signal travels across the chip between an input and an output on opposite sides of the chip. Such a prior art arrangement is shown in FIG. 1 where a silicon chip 11 has a linear waveguide 12 passing in a straight line between an input 13 and an output 14 on opposite faces of the chip. The chip has end faces, or facets, 15 and 16. The direction of the waveguide 12 is inclined to the normal direction for each of the end faces 15 and 16 so as to reduce optical reflections at the end faces of the chip to reduce back reflections. Anti-reflective coatings may be formed on the faces 15 and 16 to reduce reflection. The entire optical path between the faces 15 and 16 is electrically pumped to provide optical gain. The geometry of the waveguide 12 may be chosen to ensure that the same gain is achieved for both TE and TM polarisations. When the chip 11 is mechanically mounted in a recess in an optoelectronic integrated circuit as shown in FIG. 1, problems arise in achieving axial location of the waveguide 12 relative to connecting waveguides in the surrounding integrated circuit. In FIG. 1 the chip 11 is mounted in a recess 17 formed in an optoelectronic integrated circuit 18. The circuit 18 has an input waveguide 19 for optical communication with the input 13 of waveguide 12. The circuit 18 has an output waveguide 20 arranged to receive light from the output 14 of the waveguide 12. The waveguides 19 and 20 are each straight and aligned with waveguide 12. To achieve efficient coupling between the waveguide 12 and the waveguides 19 and 20 it is necessary for the ends of the waveguide 12 to be in very close physical proximity to the connecting waveguides 19 and 20. Typically the close physical proximity should result in a gap of less than 1 xcexcM. It is however difficult to form the chip 11 with accurate dimensions as the facets are formed by mechanical cleaving thereby making it difficult to achieve desired precision in the location of the faces 15 and 16. The faces 15 and 16 may produce smooth vertical facets by fracturing along a crystalline plane thereby producing a high quality face but the precise location of the fracture may be indeterminate. While plasma etching may be used to form accurately located facets, they are of less good optical quality.
FIG. 1 illustrates the effect of the optical amplifier chip being too short for the recess in which it is located. In this case the chip is located in position in the recess 17 by location of the input face 15 against an adjacent face of the recess 17. The chip is also located along an adjacent face 22 of the recess 17. However, the output face 16 of the chip is separated from a wall 23 of the recess thereby causing a gap 24 between the output end of the waveguide 14 and the waveguide 20. If the chip is cleaved too long it will not fit in the recess.
It is an object of the present invention to provide an improved semiconductor optical amplifier which reduces the problems of efficient optical coupling with surrounding optical circuits.
The invention provides a semiconductor optical amplifier comprising a semiconductor device having a plurality of flat edge faces, at least one waveguide with an input and an output on the semiconductor device, at least part of the waveguide being formed of active semiconductor material and said input and output being located on the same or adjacent edge faces of the device.
Preferably the input and output are formed on the same edge face.
Preferably the waveguide extends in a straight line adjacent each of said input and output.
Preferably the waveguide adjacent the input and/or output extends as a straight line inclined at an angle to the normal at the edge space in which the inlet and/or outlet is formed.
In one embodiment the waveguide has two portions, one portion leading from the input and the other portion leading to the output, said two portions being optically linked at a reflector on the device arranged to reflect light from said one portion into said other portion.
Preferably the semiconductor device has an elongated alignment edge formed at an angle to the normal at the edge face in which the input and/or output is formed.
Preferably the alignment edge is formed as a shoulder extending partway through the thickness of the semiconductor device.
The alignment edge may be formed by plasma etching.
Preferably said input and output are each formed in a cleaved facet of the semiconductor device.
The invention includes a semiconductor optical amplifier as aforesaid mounted in a recess on a supporting member having optical communication paths communicating with said input and output.
Preferably the optical communication paths are respective optical waveguides.
Preferably the supporting member is an integrated circuit optoelectronic device.
The recess in the supporting member has a first locating wall engaging an edge face in which said input or said output is formed.
Preferably the recess in said supporting member has a second locating wall engaging an alignment edge of the semiconductor device.
An electrical pumping circuit may be connected to an active portion of the waveguide in the semiconductor device.